Radiation induced, allogeneic bone marrow chimeric mice established in a laminar flow tent environment demonstrate different states of chimerism as judged by the H-2 type of the lymphocytic elements. The reactivity of splenic lymphocytes in mixed-lymphocytic cultures from chimeric mice indicated that there is no state of tolerance established. In contrast, it appears as if the lymphocytes are highly reactive to recipient H-2 antigens when cultured in vitro.